The present invention relates to mechanically operated safety switches, and more particularly to mechanically operated one-part switches, such as emergency stop switches (“mushroom buttons”), manual enabling buttons, position or limit switches, and other safety switches mechanically operated by means of levers, rollers, hinges or the like.
Prior art safety switches of this type are often used in automated installations in order to bring the installation into a safe state if a dangerous situation occurs. This may be, for example, that the installation is switched off when an emergency stop switch is manually operated. Depending on the application, however, is also possible that a safe inactive position is adopted or a partial function of the installation is blocked. Examples of safety switches of this type are disclosed by DE 38 41 458 C1, DE 199 02 910 A1, DE 199 02 919 A1, DE 199 37 947 A1 or DE 34 30 090 A1.
The prior art safety switches known heretofore share the common feature that they have a mechanically operated actuator, the respective position of which is monitored by means of a detector. In practice, the detectors until now have been formed as electromechanical switching elements, the contacts of which are opened or closed by the movable actuator. For safety switches, so-called positive opening arrangements are usually used, ensuring opening of the safety switch even when there is a malfunction in the safety switch itself, as for instance in the case of the contacts sticking in the closed position. These safety switches, particularly emergency stop switches, can be produced quite inexpensively and reliably on account of the many years of experience and on account of the high numbers involved.
In practice, however, “faith” in the functional reliability of the safety switches is nevertheless limited. For example, in the case of production installations for the automobile industry, there is a requirement that all the emergency stop switches must be manually operated at least once or twice a year in order to check their functional reliability. It is easy to see that, in the case of extensive, large installations with numerous emergency stop switches, these functional checks take up a certain amount of time and consequently a certain amount of effort. Therefore, apart from the low production costs for an emergency stop switch, there are over the service life of the installation in which the emergency stop switch is used operating costs for the regular functional checking.
DE 199 09 968 A1 discloses a safety switch for electrically-controlled machines in which the position of the movable actuator is to be evaluated by means of a number of fork light barriers. Alternatively, inductive, capacitive or pressure-sensitive, for example piezoelectric, position sensing devices are also proposed, but without any further explanation. The object of these measures is to avoid a complex mechanism for the prevention of unallowed switching sequences and, at the same time, achieve a high functional reliability.
DE 100 23 199 A1 discloses a safety switch for electrical machines which has a force or pressure sensor, so that it is possible to distinguish between a desired or intended operating state and a state of panic.
Furthermore, DE 100 37 003 A1 discloses a key-operated switch which is intended for checking access authorization in computer-aided control devices of machines, installations or the like. Instead of a mechanical key, use of a transponder is envisaged to establish the access authorization.
The last-mentioned key-operated switch is also shown in a book with the title “Sicherheitsschaltgeräte” [Safety Switching Devices], published by Verlag Moderne Industrie as volume 232 of the series “Bibliotek der Technik”, ISBN 3-478-93234-3. As a general outlook for the technical development in this area, mention is also made here of the idea of integrating in an enabling switch a transmitter which, when operated, transmits a unique code to an assigned receiver by radio data communication, whereby the previously often troublesome cables for enabling switches are to be rendered superfluous.
Furthermore, EP 0 229 247 A2 or EP 0 968 567 B1 discloses signal transmitters operating contactlessly (that is per se two-part, non-mechanically operated signal transmitters), which also operate on the transponder principle. These contactless signal transmitters are to be used, inter alia, for the monitoring of guard doors.